Today there are different joining methods for joining metal parts (metal objects or metal workpieces) that are made of metallic elements, which metallic elements include various elemental metals as well as various metallic alloys. The metal parts in question have, by virtue of the metallic elements or alloys they are made of, a melting temperature of at least 1100° C., which means that the metal parts cannot be made of e.g. pure copper, pure aluminum or various aluminum-based alloys. Some examples of metal the metal parts may be made of are typically iron-, nickel- and cobalt-based alloys.
One common method for joining such metal parts is welding which is a method where the metal in the metal part with or without additional material is melted, i.e. a cast product is formed by melting and subsequent re-solidification.
Another joining method is brazing which is a metal-joining process where a filler metal first is applied on at least one of two metal parts to be joined and then heated above its melting point and distributed between the metal parts by capillary action. The filler metal is brought above its melting temperature, typically under protection by a suitable atmosphere. The filler metal then flows over the metal parts towards contact points where it forms joints.
Generally, when brazing, a filler metal is applied in contact with a gap or a clearance between the metal parts to be joined. During the heating process the filler metal melts and fills the gap to be joined. In the brazing process there are three major stages where the first stage is called the physical stage. The physical stage includes wetting and flowing of the filler metal. The second stage normally occurs at a given joining temperature. During this stage there is solid-liquid interaction, which is accompanied by substantial mass transfer. A small volume of the metal parts that immediately adjoins the liquid filler metal either dissolves or is reacted with the filler metal in this stage. At the same time a small amount of elements from the liquid phases penetrates into the solid metal parts. This redistribution of components in the joint area results in changes to the filler metal composition, and sometimes, the onset of solidification of the filler metal. The last stage, which overlaps the second, is characterized by the formation of the final joint microstructure and progresses during solidification and cooling of the joint. The volume of the metal parts that adjoins the liquid filler metal is very small, i.e. the joint is formed to the largest extent by the filler metal. Generally, when brazing, at least 95% of the metal in the joint comes from the filler metal.
Another method for joining two metal parts (parent materials) is transient liquid phase diffusion bonding (TLP bonding) where diffusion occurs when a melting point depressant element from an interlayer moves into lattice and grain boundaries of the metal parts at the bonding temperature. Solid state diffusional processes then lead to a change of composition at the bond interface and the dissimilar interlayer melts at a lower temperature than the parent materials. Thus a thin layer of liquid spreads along the interface to form a joint at a lower temperature than the melting point of either of the metal parts. A reduction in bonding temperature leads to solidification of the melt, and this phase can subsequently be diffused away into the metal parts by holding at bonding temperature for a period of time.
Joining methods such as welding, brazing and TLP-bonding successfully joins metal parts. However, welding has its limitations as it may be very expensive or even impossible create a large number of joints when they are hard to access. Brazing has also its limitations, for example in that it sometimes it is hard to properly apply or even determine a most suitable filler metal. TLP-bonding as advantageous when it comes to joining different material but has its limitations. For example, it is often hard to find a suitable interlayer and the method is not really suitable for creating a joint where a large gaps is to be filled or when a relatively large joint is to be formed.
Thus, many factors are involved when selecting a certain joining method. Factors that also are crucial are cost, productivity, safety, process speed and properties of the joint that joins the metal parts as well as properties of the metal parts per se after the joining. Even though the aforementioned methods have their advantages, there is still a need for a joining method to be used as a complement to the present methods, in particular if factors like cost, productivity, safety and process speed are taken into account.